Conventionally, various types of displacement sensors which detect the amounts of displacement when an operator operates a main body have been devised. Operations performed on a main body include bending the main body, twisting the main body and pushing a predetermined surface (a touch surface) of the main body. Various types of touch sensors which detect the amounts of pushing upon touching when an operator touches a flat operation surface by the finger have been devised as touch sensors whose touch surfaces are pushed. For example, Patent Document 1 discloses a touch input device which has a flat pressure-sensitive sensor. A piezoelectric sheet using a piezoelectric body as a material is generally used for such a pressure-sensitive sensor. Further, this piezoelectric sheet is usually used for the various types of current displacement sensors.
The piezoelectric sheet generates a charge corresponding to the amount of displacement, and therefore the amount of displacement (the amount of bending, the amount of twisting or the amount of pushing) is detected by configuring a piezoelectric sensor by forming electrodes opposing both surfaces of the piezoelectric sheet, and detecting voltages from generated charges.
Patent Document 1: JP 5-61592 A
However, a piezoelectric body used for a piezoelectric sheet generates the amount of charge proportional to the amount of displacement, and, when the piezoelectric body continues maintaining the same amount of displacement, the piezoelectric body generates no new charge after generating a certain amount of charges. That is, when an output of the piezoelectric sensor is connected to a load having a fixed input impedance and is observed as a voltage, this voltage instantaneously changes from an initial reference voltage to a voltage value proportional to the amount of displacement, and subsequently returns to the reference voltage without being maintained at a fixed voltage.
According to the conventional configuration, it is not easy to precisely detect (measure) the amount of displacement proportional to charges generated by a piezoelectric sensor due to characteristics of the piezoelectric sensor.